Add documentation for use with Weechat-Android

[riotbib]

This pull request adds documentation for use with Weechat-Android.

Being able to easily look up the usage is handy when setting up a new phone.

The mention of the ?onlyURL flag under features is optional, IMHO.

[riotbib]

Thanks for the feedback. I pushed a new commit, minding your input.

I added one detail: The name of the setting for file sharing in Weechat-Android, thus it's easier to look up.

[oxzi]

Thanks a lot :}